(submitted for publication to Science, May 11, 1998) A strategy for the design of molecules with large two-photon absorption cross-sections, (, has been developed. The strategy is based on the concept that symmetrical charge transfer, from the ends of a conjugated system to the middle, or vice versa, upon excitation is correlated to enhanced values of (. We show that newly synthesized bis-styrylbenzene derivatives with donor-(-donor, donor-acceptor-donor, and acceptor-donor-acceptor structural motifs exhibit exceptionally large values of (, up to 150 times that of trans-stilbene. Quantum chemical calculations performed on these new molecules indicate that substantial symmetric charge redistribution occurs upon excitation, and provide ( values in good agreement with experiment. The combination of large ( and high fluorescence quantum yield of triplet yield exhibited by molecules developed here offers potential for unprecedented brightness in two-photon fluorescent imaging or enhanced photosensitivity in two-photon sensitization, respe ctively.